There are at least two main differences between 440C and CPM S30V that I am aware of:
1. CPM S30V is a wear and corrosion resistant martensitic tool-steel grade manufactured by using Crucible Particle Metallurgy (CPM, hence the name) process (i.e., CPM S30V it is a PM grade), whereas 440C is a conventionally ingot-cast steel. The microstructure of a PM grade is finer and more uniform compared to that of a conventional steel grade such as 440C.
2. The alloying content of CPM S30V is significantly different from that of 440C (which is in essence just Fe-1C-17Cr). The nominal chemical composition of CPM S30V is as follows (in wt. %):
C = 1.45
Cr = 14
V = 4
Mo = 2
The presence of vanadium in CPM S30V results in the precipitation of very hard vanadium-rich MC primary carbides, whose hardenss is about 3000 HV. The presence of 2 wt. % Mo contributes to the secondary-hardening effect, i.e., the higher hardness if tempered at higher tempering temperatures.
440C contains only chromium-rich primary carbides (and they are noticably less hard than vanadium-rich primary carbides).
Finally, the PM microstructure improves steel's toughness characteristics for a given chemical composition.